1. Field of the Invention:
The present invention relates to an apparatus and method for manufacturing flat glass in which the glass is formed while being supported on the surface of the pool of molten metal. More particularly, this invention relates to a method for introducing protective atmosphere gases into a headspace of the chamber in which the glass is formed.
2. Brief Description of the Prior Art:
In a variety of processes a continuous sheet of ribbon of glass is formed while supported on and advancing along the surface of a pool of molten metal. In general, a protective, nonoxidizing atmosphere is provided in a headspace above the advancing glass and the molten metal supporting it in order to prevent the oxidation of the molten metal and consequent contamination of the glass. Several patents illustrating the use of protective atmosphere to protect molten metal in this environment are U.S. Pat. No. 3,241,937 to Michalik et al; U.S. Pat. No. 3,337,322 to Taylor; U.S. Pat. No. 3,356,476 to Gulotta; U.S. Pat. No. 3,551,126 to Sacrez et al and U.S. Pat. No. 3,630,701 to Javaux et al. All of these patents show the introduction of protective atmosphere gases into a headspace overlying an advancing body of glass that is being formed into a continuous sheet and over a pool of molten metal that supports the advancing glass. The headspace into which the protective atmosphere gases are introduced is defined by the side walls and end walls of the forming chamber and by some overlying ceiling or roof spaced above the advancing glasses and its supporting metal.
As shown in U.S. Pat. No. 3,468,869 for example, electrical heating elements for controlling the temperature of the forming chamber may be mounted in a refractory ceiling above the glass. The ceiling, which extends over the advancing glass and its supporting metal, may be separated from and enclosing within an outer shell or casing as shown in U.S. Pat. No. 3,721,543 to Classen et al. Such an arrangement is shown generally for electrically heated enclosures in U.S. Pat. No. 2,543,708 to Rice et al. The patent of Rice et al shows a double-walled furnace having an inner wall like the ceiling of a glass forming chamber made of refractory material surrounded by an outer metallic wall or casing. Electric heaters extend through the inner wall to maintain the temperature within the interior space.
According to the disclosure of Rice et al, a suitable gaseous mixture for introduction into the inside chamber defined by the inner wall is introduced into the space between the inner and outer walls and allowed to diffuse through the inner wall into the furnace. While passing through the space between the inner and outer walls, such a gas inherently removes some heat from the electric heaters and their connections to conductors located within the space between the inner wall and outer wall of the disclosed furnace or chamber.
In the past it has been common to introduce protective gases of differing compositions into different portions of the space or plenum above the ceiling of a forming chamber so that controlled amounts of a reducing gas would flow down through the ceiling at different locations along the length of the chamber. For example, it has been known from Taylor in U.S. Pat. No. 3,337,322 to introduce a protective gas comprising from 0.25 to 3.0 percent hydrogen in nitrogen into a plenum near the inlet end of a chamber while introducing a protective gas comprising from 3.0 to 10 percent hydrogen in nitrogen into the plenum near the outlet end of the chamber. These gases of differing constituent concentrations generally flow downwardly through spaces and openings between adjacent refractory pieces of which the ceiling is comprised. The rate of gas introduction and its flow through the ceiling are largely dictated by a requirement that there be little or no cycling of gas flow upwardly, as well as downwardly, through the ceiling and by a requirement that electrical apparatus or the like located in the space or plenum above the ceiling be kept relatively cool.
Because of the required reducing character of the gases filling the headspace of a forming chamber beneath its ceiling and in the vicinity of its pool of molten metal, all of the gases entering the chamber in a method such as that of Taylor must contain a reducing constituent such as hydrogen. This is so then for all gas introduced into the chamber even if its principal function is to provide cooling for electrical equipment and a pressure seal for the chamber. This is wasteful of hydrogen and may in some instances be detrimental, as for example in the flames or "sting out" associated with the use of excessive hydrogen.
This invention provides for a more conservative use of hydrogen or other reducing gases and for the separation of the functions of introduced protective gases into their component functions which may then be individually controlled and optimized.